Chromium plating process



Patented June 21, 1932 UNITED. STATES PATENT OFFICE DWIGHT r. nwme, ornear msma-nrcmeaiw CHBOMIUH PLATING PROCESS No Drawing.

The invention relates 'to electrolytes forelectrodeposition of chromiumand to a process for electroplating chromium in a form suitable forcommercial requirements.

In my copending application Serial No. 126,094, filed J uly 30, 1926, Ihave-described a method for rendering a solution of chromic acidsuitable for electrodeposition of chromium by the addition of certaincompounds of 4 metals having oxidizing and reducing propof chromium andthe electrolyte is renderedsuitable for depositing chromium by theaddition of a cerium compound, such for example as cerium, oxalate,cerium carbonate, etc. Although chromium may be plated from suchsolutions the results at times are I irregular, due to the formation ofa brownish film in spots upon the surface of the chromium. Where suchareas are found the deposits of chromium seem to be thinner and undercertain conditions the film, upon drying the plated object, isquiteadherent. and bufling is required for its removal. The formation of someof thefilm may be prevented to a large extent by washing the objects tobe plated in a certain manner, but I have discovered another method toprevent its formation which consists in adding to the electrolytecertain additional substances. While there are a large number ofdifferent "sub stances which have the property of prevent- (a ing thebrownish film formation, I have found it most advantageous to usea saltof a metal having alow. hydrogen over-voltage. The

' quantity of this material which-should be used can not be well statedin terms of weights and volumes because it is-a function of such factorsas current density, temperature of Application filed January 3, 927.Serial No. am.

bath, concentration of chromic acid, proportion of cerium oxalate orother compound added to the chromic acid, the anode material and theshape of the object to be plated.- Therefore the amount of thefilm-preventing substance which should be added tothe elec-" trolyte canbest be found'by trial, :although it may be stated briefly that theamount used should be that quantity which is just necessary to preventthe formation of the film when the operator is working with a cleanpiece of work, such as an object of brass, steel or nickel. Ifsubstantially more than this amount is added, the efiiciency of theplating process is somewhat lowered. As an example of some of thesubstances which have been found to prevent the formation of thebrownish film are the following: hydrofluo-' silicic acid,hydrofluoboric acid, or their soluble salts, sulphites or persulphates.7 As an example of a specific electrolyte suitable for commercialrequirements and the process of preparing the solution, the fol lowingformula is 'givenf Add to a liter of water 200 to 300 grams ofcommercial chromic acid. When this has dissolved, 10 grams of commercialcerium oxalate is added, with stirring, and dissolved. This solution isnow treated with a dilute solution of sodium fluosilicate about 1 gram30 per 100 cc. solution until upon making a trial under the best currentdensity by plating a" brass-or nickel object (area about 1 squaredecimeter) no brown film is formed. The

temperature of the bath may be about 20 Instead of incorporating thecerium oxalate addition-agent in a chromicacid solution it'may be usedin other chromium-ca ing electrolytes where the chromium is in t ehexavalent state substantially free from trivalent chromium, Forexample, under certain conditions good deposits can-be obtained from asolutioncontaining per liter of solution 290 grams sodium-chromate'Nacrolmmo and 10 cerium oxalate.

Whilein the above formma Ihzwesuggested the useofcerium oxalataas a-sustance for rendering the bath p suitable for-electroplating, it is to beunder-' stood'thatmy invention can also be carried out: by adding thefilm-preventing substance [to a chromic acid' bath having incorporatedtherein other compounds of cerium such as theoxides,earbonates,sulphates, etc. and in the broader aspects of theinyentionthe film- 'preventing compound may be addedto any" positbeing'of uniform thickness, bright andlustrous and free from anybrownish film. It'is believed that-one of the reasons .why

go-sodium fluosilicate and other fluosilicates have such abeneficialactionon the electrolyte is due to the fact that they are inorganiccolloidal substances. It is believed essential. that the substance beinorganic in character abrather than organic because the presence ofchromic acid in the electrolyte introduces such strong oxidizingproperties that organic matter present is destroyed.

I In carrying out the process of electrodep 3Q osition ofchromiuimutilizing the improved electrolyte above described, it has beenfound very advantageous to accurately determine:

the amount of current in the, working circuit, especially when thebathisbeing used 3 for objects of differentshapes or materials. It istherefore desirable to-have .an ammeter arranged in the workingcircuitand byusing the ammeter'I have discovered. amethod bywhich theefliciency of the electroplating 40 process maybe increased and theproper current density determined for any particular object which is tobe plated. My method con- '.sists in placing the object intheelectrolyte with a small current flowing in the working;

circuit and gradually increasing the efi'ec-' tive voltage on the bathby rheostats or other pound Shaving valent-state substantially free fromtrivalent is desired to obtain bright polished chromium surface it ispreferable to have a polished under surface on whichthe deposition takesplace to avoid subsequent smoothing of the extremely hard chromiumplate.

hat I claim as my invention is': 4 1. Anelectrolyte forelectrodep'osition' of chromium comprising a solution containingprincipally chromic acid, together with small amounts of cerium oxalateand sodium hydrofluosil-icate.

principally chromic acid and a relatively.

small amount of a cerium compound.

3. Anelectrolyte for electrodeposition of chromium comprising. a.solution containing principally chromic. acid and a relatively smallamount of cerium' oxalate."

4.'An electrolyte for electrodeposition'of chromium comprising asolution containing as-the principal ingredient chromic acid, arelatively. small amount of a cerium compound and a solublefluosilicate.

v5. An electrolyte for electrodeposition ofchromium comprising .asolution containing having oxidizing and reducing properties -2. Anelectrolyte for electrodeposition of chromium comprising a solutioncontaining relatively small amount of a cerium compound and sodiumfluosilicate.

[-6. An electrolyte for electrodeposition ofas the principal ingredient.a chromium comthe chromlum 1n the hexachromium and a serium compound.Intestimony whereof I aflix my signature.

DWIGHT T. EWING.

suitable means and carefully noting'the resulting increase of currentflowing throu h thef circuit. It has been found that as the voltage isadually. increased the current .als o 1s gra ually' and uniformlyincreased -'unt1l a certain criticalpoint is reached where there isasudden and appreciable drop in the amperage asin'dicatedby the ammeter.It has been found thatwhen the object isplated with the currentcorresponding tothe critical,

point ora current just slightly greater than the crltlcaljpomt, a verysatisfactory deposit of. chromium .is obtained and the efii--' 00 ciencyof -th e; ath is higher than; with other current dens1ti'es.'-

. It fis'to be understood that a. electroplatingprocess asabove.described can be carried 2' out onsteel, copper, or nickel surfaceswhich. 6 are either pohshed or unpolished,;but if it chromium comprisinga solution containing

